Gilbert HAVC Systems – Bigger Isn’t Always Necessarily Better

Two problems we commonly see with Gilbert HVAC installations are improperly sealed ductwork and improperly sized systems. Generally, systems tend to be oversized which lead to short runtimes. When performing retrofits, customers should be sure that their Gilbert HVAC service providers are using the Air Conditioning Contractors of America (ACCA) Manual J and Manual S. Replacing a system based on the existing unit’s name plate is not good enough. Using a “rule of thumb” based on a home’s square footage is not good enough either. These methods might provide a good starting point or first estimate, but they’re not good enough to properly size your system.

More than reading the nameplate of the existing system, correct sizing requires that the contractor consider factors like the local climate, the shape and orientation of the house, air infiltration rates, and window area, location and type. Manual J, “Residential Load Calculation,” published by the ACCA, is the recommended method for use in the United States and takes into account these and many other considerations. You should make sure that the contractor is trained in and follows the procedures outlined in Manual J.

Bigger is always better. Right? Maybe when it comes to your paycheck or the length of your vacation but not necessarily when it comes to the size of your HVAC system. Manual S, Residential Equipment Selection, is the companion to Manual J. An oversized system will heat and cool just fine, but it will run for such a short time and cycle so frequently that it will do a poor job conditioning the air.

Short runtimes lead to poor efficiency. In the summer, the system will do a fine job cooling the air but will offer poor humidity control which can lead to mold and mildew growth which in turn can lead to allergic reactions and asthma attacks. Short runtimes result in more air stratification and poor air mixing in the house resulting in large temperature swings resulting in a dramatic reduction in comfort. And not only will the oversized unit cost you more initially, short runtimes and frequent cycling will lead to early compressor and fan failure.

When replacing a HVAC system, make sure that your ACCA Manual J and Manual S calculation include improvements to the thermal envelope of the house. Properly insulating and air sealing an attic can make a significant difference in the size of the HVAC system necessary comfortably condition a home’s air.

I frequently talk with homeowners who want to replace their ten year old HVAC systems in an attempt to reduce outrageous power bills. If the system is working, more often than not all that is needed is a comprehensive HVAC tune up, duct sealing, and air sealing. That can save the homeowner thousands of dollars as well as achieve the desired reduction in monthly power bills.

Personally, I would rather have a 13 or 14 SEER system with well installed and well sealed ductwork than an 18 Seer system with most of the existing ductwork I have seen. It is not unusual for a duct system to test at 20% to 30% air leakage and 50% or more air leakage is not unheard of. That is blowing up to half of your conditioned air into an unconditioned crawlspace or attic – a total waste of energy! Never replace an HVAC system without first replacing or repairing bad ductwork. Good ductwork is a prerequisite to a new high efficiency Gilbert HVAC system since it is often a reduction in air-leakage that allows the system to be downsized in the first place.

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